1. Technical Field
The present invention relates to oscillators in general, and, in particular, to an ovenized oscillator for generating a reference frequency signal in electronic equipment. Still more particularly, the present invention relates to an apparatus for ensuring ovenized oscillator to generate a stable reference frequency signal.
2. Description of Related Art
Oscillators are generally utilized to provide a reference frequency signal for other electronic components. An oscillator typically has a quartz crystal (or resonator) along with other electronic compensation circuitry to generate a reference frequency output signal. Since the reference frequency output signal from an oscillator may change as the temperature of the oscillator changes, various methods are required to be utilized to stabilize the reference frequency output signal of the oscillator. For example, ovens can be furnished within oscillators to heat certain sensitive circuits that are isolated from the ambient in order to obtain a more stable reference frequency output signal.
Ovenized oscillators usually contain a heater and a temperature sensor along with a temperature controller to control the heater. The temperature controller holds the crystal and other critical circuitry at a constant temperature. A good temperature controller can provide a steady current that changes with the ambient temperature to hold the oven at a precise set-point, usually about 10 degrees above the highest expected ambient temperature. Temperature induced frequency variations can be greatly reduced by an amount approaching the thermal gain of the oven. The crystal for the oven is selected to have a “turning-point” at or near the oven temperature to further reduce its sensitivity to temperature. The combination of the high oven gain with operation near turning point yields frequency stabilities as good as 0.0001 ppm over a temperature range that would otherwise cause the crystal to change by 10 ppm.
The frequency stability of a single ovenized oscillator can be further improved by using two ovens. In a dual-oven configuration, one insulated enclosure is placed inside of another insulated enclosure with a proportionally controlled heater assembly for each. In order to best maintain temperature control of the assembly under varying ambient conditions, it is necessary to maintain a differential of about 10 degrees Celsius between the highest ambient temperature to be experienced and the set point of the outer oven. Another 10 degrees Celsius differential is then required between the set points of the outer oven and the inner oven. Thus, the total temperature rise above ambient between the crystal/resonator and the outside of the outer oven can be more than 20 degrees Celsius.
Although dual-oven oscillators can generate highly stable frequency output signals, they are also very expensive. Consequently, it would be desirable to provide an improved apparatus for providing oscillator frequency stability.